This invention relates to a method for determining the radius of a bending die for use with a bending machine for bending a part and an associated apparatus. The invention also includes an improved bending machine and a method of bending a part.
Bending machines for bending parts, such as aluminum extrusions, are well known. In order to bend a part to a desired final shape, a bending die is used. Bending dies are used in both rotary draw bending and in stretch die bending.
In designing extrusions for use in, for example, automobiles, the part designer usually specifies a final bend angle and a radius of curvature for the part. These specifications are given to the bending machine designer so that he can design a bending die and determine process parameters (i.e., amount of torque, tension in the part) that will produce a part that meets the part designer's specifications. The bending machine designer must determine a bending die radius and must also calculate how much to "overbend" the part in order to compensate for "springback" when the part is unloaded from the bending machine.
Design of the bending die and control of the process parameters traditionally relied on experience and intuition of the bending machine designer and the bending machine operator. Because of this, there were relatively involved "trial-and-error" preproduction experiments to determine die geometrics and process parameters. These preproduction experiments were not only time consuming and imprecise, but also involved expensive die proofing and die modifications in many cases.
What is needed, therefore, is a method that facilitates designing a bending die that can be used on a bending machine for bending a part to a desired shape without the need for extensive experimentation, die proofing and die modification.
Furthermore, once the bending die is designed, and the process parameters determined, there is also a need for an improved bending machine that controls the bending process based on the previous method of determining the bending die radius.
Typically, rotary die bending machines include a stationary portion and a rotary bending die portion which rotates relative to the stationary portion. The part to be bent is clamped to both the stationary portion and the rotary die. After clamping, the rotary bending die is rotated relative to the stationary portion. Because the part is clamped to the rotary die, the part is drawn along with the bending die causing a bend to be formed in the part. See, for example, U.S. Pat. No. 3,821,525, which is expressly incorporated herein by reference.
As is well known, after the part has been bent and unloaded from the bending machine, the part will "springback" to a different bend angle. In order to obtain a desired final bend angle, this springback must be compensated for by "overbending" the part. Determining the amount of overbending to produce a final desired unloaded bend angle typically involved operator intuition, experience and "feel".
U.S. Pat. No. 5,050,089 discloses an online, automatically compensated tube bender which compensates for springback. This patent discloses a sensor for measuring the bending moment and calculating the springback angle of the tube being bent as a function of the bending moment determined during bending. This patent, however, is limited to tubular type of parts and only takes into account the bending moment and is not useful for parts having complex cross-sectional shapes.
What is also needed, therefore, is an improved bending machine and an associated method which facilitates bending of parts having a wide range of mechanical properties and which has any cross-sectional shape.